Light olefins, such as ethylene, propylene, butylenes, and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, C2-C4 light olefins are produced by cracking petroleum refinery streams, such as C3+ paraffinic feeds. In view of limited supply of competitive petroleum feeds, production of low cost light olefins from petroleum feeds is subject to waning supplies. Efforts to develop light olefin production technologies based on alternative feeds have therefore increased.
An important type of alternative feed for the production of light olefins is oxygenates, such as C1-C4 alkanols, especially methanol and ethanol; C2-C4 dialkyl ethers, especially dimethyl ether (DME), methyl ethyl ether and diethyl ether; dimethyl carbonate and methyl formate, and mixtures thereof. Many of these oxygenates may be produced from alternative sources by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastic, municipal waste, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as economical, non-petroleum sources for light olefin production.
The preferred process for converting an oxygenate feedstock, such as methanol, into one or more olefin(s), primarily ethylene and/or propylene, involves contacting the feedstock with a crystalline molecular sieve catalyst composition. Crystalline molecular sieves all have a three-dimensional, four-connected framework structure of corner-sharing [TO4] tetrahedra, where T is one or more tetrahedrally coordinated cations. Examples of well known molecular sieves are silicates, which comprise [SiO4] tetrahedral units; aluminosilicates, which comprise [SiO4] and [AlO4] tetrahedral units; aluminophosphates, which comprise [AlO4] and [PO4] tetrahedral units; and silicoaluminophosphates, which comprise [SiO4], [AlO4], and [PO4] tetrahedral units.
Molecular sieves are typically described in terms of the size of the ring that defines a pore, where the size is based on the number of T atoms in the ring. Other framework-type characteristics include the arrangement of rings that form a cage, and when present, the dimension of channels, and the spaces between the cages. See van Bekkum, et al., Introduction to Zeolite Science and Practice, Second Completely Revised and Expanded Edition, Volume 137, pp. 1-67, Elsevier Science, B. V., Amsterdam, Netherlands (2001). Among the molecular sieves that have been investigated for use as oxygenate conversion catalysts, small pore aluminophosphates and silicoaluminophosphates (having a pore size less than 5 Å), such as SAPO-34, have shown particular promise. SAPO-34 belongs to the family of molecular sieves having the framework type of the zeolitic mineral chabazite (CHA).
Also reported as having activity in the conversion of oxygenates to olefins are intergrowths of CHA framework-type molecular sieves with AEI framework type molecular sieves, such as RUW-19 as disclosed in U.S. Pat. No. 6,334,994 and EMM-2 as disclosed in U.S. Pat. Nos. 6,812,372 and 6,953,767.
For example, U.S. Pat. No. 4,499,327 discloses a process of making light olefins containing 2 to 4 carbon atoms which comprises contacting a feedstock comprising one or more of methanol, ethanol, dimethyl ether, diethyl ether and mixtures thereof with a silicoaluminophosphate (SAPO) molecular sieve having a specified unit empirical formula in the as-synthesized and anhydrous form. Preferred SAPOs are those that have pores large enough to adsorb xenon (kinetic diameter of 4.0 Å), but small enough to exclude isobutane (kinetic diameter of 5.0 Å), with SAPO-34 being particularly preferred.
In addition to framework topology, one of the factors that frequently affects the efficacy of a molecular sieve for use in the conversion of oxygenates-to-olefins is the crystal size and crystal size distribution of the molecular sieve particles. For example, U.S. Pat. No. 5,126,308 reports that an aluminophosphate catalyst having the formula (ELxAlyPz)O2, wherein EL is a metal selected from the group consisting of silicon, magnesium, zinc, iron, cobalt, nickel, manganese, chromium, and mixtures thereof, has improved catalyst life and decreased by-product formation in oxygenate conversion reactions when at least 50% of the catalyst particles have a particle size smaller than 1.0 μm and no more than 10% of the particles have particle sizes greater than 2.0 μm. In particular, the Examples of the '308 patent show that SAPO-34 having a median particle diameter, expressed as a mass distribution, of 0.71 micrometers and 90% of the total sample mass having a median particle diameter<1.2 micrometers had a longer life and produced less C3 by-product in methanol conversion than SAPO-34 having a median particle diameter of 0.90 micrometers, 90% of the total sample mass having a median particle diameter<3.0 micrometers and 10% having a median particle size<0.5 micrometers.
The synthesis of aluminophosphate and silicoaluminophosphate molecular sieves involves preparing a reaction mixture by mixing a variety of starting materials including a source of water, a source of phosphorus, a source of aluminum, optionally, a source of silicon, and at least one organic directing agent for directing the formation of the desired molecular sieve. The resultant mixture is then heated, normally with agitation, to a suitable crystallization temperature, typically between about 100° C. and about 300° C., and then held at this temperature for a sufficient time, typically between about 1 hour and 20 days, for crystallization of the desired molecular sieve to occur.
According to the present invention, it has now been found that controlling the temperature during mixing of the starting materials and, in particular, ensuring that the temperature of the starting materials is kept between 25° C. and 50° C., preferably between 30° C. and 45° C., until formation of the reaction mixture is complete, is important in synthesizing aluminophosphate and silicoaluminophosphate molecular sieves having a homogeneous crystal size distribution. One way of measuring the homogeneity of crystal size distribution is by determining crystal size span, wherein the crystal size span is defined as:(d90−d10)/d50where d10, d50, and d90 are the maximum particle sizes of 10%, 50%, and 90% respectively of the molecular sieve particles. In particular, it is found that by maintaining the temperature between 25° C. and 50° C., preferably between 30° C. and 45° C., until gel formation is complete it is possible to produce CHA framework-type aluminophosphate and silicoaluminophosphate molecular sieves in which the crystal size span is less than 1.
In our issued, commonly assigned U.S. Pat. No. 7,090,814, we have disclosed a method of synthesizing a novel silicoaluminophosphate molecular sieve, in which a synthesis mixture is prepared by combining a source of phosphorus and at least one organic directing agent; and then the combination of the phosphorus source and organic directing agent is cooled to a temperature of less than or equal to 50° C., preferably less than or equal to 30° C., prior to introducing a source of aluminum into the combination. After addition of a source of silicon, the synthesis mixture is heated to a crystallization temperature of between about 100° C. and about 300° C. and the molecular sieve is recovered.